Believe a problem may be what the metric means / how is it measure / how it was obtained. So I would not place too much reliance on this information as presented.

I can assure you the UA seatbelts are not limited to individuals with waistines of 31", I would guess in reality it is closer to at least the higher-40's or higher. There is some variability as occasionally seatbelts are trimmed in length due to wear and tear.